Interleukin-2 (IL-2) has established efficacy for the treatment of metastatic or unresectable renal cell carcinoma and malignant melanoma. The effectiveness of Interleukin-2 as an anti-tumor agent likely depends upon the metabolic pathway that leads to the generation of nitric oxide from L-arginine. This study tested the hypothesis that this pathway could be augmented by increasing intracellular concentrations of glutathione by infusing N-acetylcysteine. It was hoped that N-acetylcysteine would increase the activity of IL-2 against the two tumors of interest, and that the toxicity would be acceptable.